Conventional adjustable speed drives (ASDs) are used to control centrifugal pumps in a system, typically by directly controlling the speed at which pumps operate. Often, the pumps are controlled at a speed that is intended to maintain a particular set point of a controlled system variable such as pressure or flow. However, the speed at which a pump operates and the controlled variable in the system usually have a non-linear, and often nearly unpredictable, relationship. Therefore, while the pumps may be controlled so as to maintain the controlled variable, the pumps may be operated at a speed that is more than necessary to achieve such a state. Also, because it is generally unpredictable what speed will correspond to a particular pressure or flow (especially since system conditions may change from time to time), it may take quite a bit of time for the pumps to assume a relatively steady state from startup or after a change in the set point.
This non-linear relationship between the drive output speed value and the controlled variable can make controlling and balancing one or more pumps in a system very complex. Furthermore, the system is typically dynamic and continuously changing depending upon the load, pump differential performance, motor performance, and power delivery performance from the drive controller. Often, a very small change in the speed of one pump may shift the entire load to another pump in the system
There have been some known systems implemented by at least one of the inventors listed in the present application in which pumps are controlled by set amount of drive current rather than by set amounts of pump speed. These systems would let the speed attain a natural value base on the set amount of drive current. However, these systems were unable to manage and load balance across multiple pumps in the same system without assuming that each pump would receive the same amount of drive current for a given set point. Moreover, these systems typically controlled the set point from a device separate from the drive controller, thereby preventing the drive controller from adjusting the set point quickly based on system feedback.